The present invention relates to halogenated, low-gel, high molecular weight isoolefin multiolefin copolymers, in particular to halogenated, low-gel, high molecular weight butyl rubbers, and halogenated, low-gel, high molecular weight isoolefin multiolefin copolymers synthesized from isobutene, isoprene and optionally further monomers, with a multiolefin content of greater than 2.5 mol %, a molecular weight Mw of greater than 240 kg/mol and a gel content of less than 1.2 wt. % and a process for their preparation.
Butyl rubber is a copolymer of an isoolefin and one or more conjugated multiolefins as comonomers. Commercial butyl comprises a major portion of isoolefin and a minor amount, not more than 2.5 wt %, of a conjugated multiolefin. The preferred isoolefin is isobutylene. Suitable conjugated multiolefins include isoprene, butadiene, dimethyl butadiene, piperylene, etc. of which isoprene is preferred. It is generally prepared in a slurry process using methyl chloride as a vehicle and a Friedel-Crafts catalyst as the polymerization initiator. The methyl chloride offers the advantage that AlCl3 a relatively inexpensive Friedel-Crafts catalyst is soluble in it, as are the isobutylene and isoprene comonomers. Additionally, the butyl rubber polymer is insoluble in the methyl chloride and precipitates out of solution as fine particles. The polymerization is generally carried out at temperatures of about xe2x88x9290xc2x0 C. to xe2x88x92100xc2x0 C. See U.S. Pat. No. 2,356,128 and Ullmanns Encyclopedia of Industrial Chemistry, volume A 23,1993, pages 288-295. The low polymerization temperatures are required in order to achieve molecular weights which are sufficiently high for rubber applications.
Raising the reaction temperature or increasing the quantity of isoprene in the monomer feed results in more poor product properties, in particular, in lower molecular weights. However, a higher degree of unsaturation would be desirable for more efficient crosslinking with other, highly unsaturated diene rubbers (BR, NR or SBR).
The molecular weight depressing effect of diene comonomers may, in principle, be offset by still lower reaction temperatures. However, in this case the secondary reactions, which result in gelation occur to a greater extent. Gelation at reaction temperatures of around xe2x88x92120xc2x0 C. and possible options for the reduction thereof have been described (c.f. W. A. Thaler, D. J. Buckley Sr., Meeting of the Rubber Division, ACS, Cleveland, Ohio, May 6-9, 1975, published in Rubber Chemistry and Technology 49, 960-966 (1976)). The auxiliary solvents such as CS2 required for this purpose are not only difficult to handle, but must also be used at relatively high concentrations.
Furthermore, it is furthermore known to perform gel-free copolymerization of isobutene with various comonomers to yield products of a sufficiently high molecular weight for rubber applications at temperatures of around xe2x88x9240xc2x0 C. using pretreated vanadium tetrachloride (EP-A1-818 476).
It is also possible to use this aged vanadium initiator system at relatively low temperatures and in the presence of an isoprene concentration which is higher than conventional (approx. 2 mol % in the feed), but, as with AlCl3-catalyzed copolymerization at xe2x88x92120xc2x0 C., in the presence of isoprene concentrations of  greater than 2.5 mol % this results in gelation even at temperatures of xe2x88x9270xc2x0 C.
Halogenated butyls are well known in the art, and possess outstanding properties such as oil and ozone resistance and improved impermeability to air. Commercial halobutyl rubber is a halogenated copolymer of isobutylene and up to about 2.5 wt % of isoprene. As higher amounts of isoprene lead to gelation and/or too low molecular weight of the regular butyl being the starting material for halogenated butyl, no gel-free, halogenated butyls with comonomer contents of greater than 2.5 mol %, a molecular weight Mw of greater than 240 kg/mol and a gel content of less than 1.2 wt. % are known.
One object of the present invention was to provide halogenated, low-gel, high molecular weight isoolefin multiolefin copolymers, in particular to halogenated, low-gel, high molecular weight butyl rubbers, and halogenated, low-gel, high molecular weight isoolefin multiolefin copolymers synthesized from isobutene, isoprene and optionally further monomers, with a multiolefin content of greater than 2.5 mol %, a molecular weight Mw of greater than 240 kg/mol and a gel content of less than 1.2 wt. %.
Another object was to provide a process for the production of halogenated low-gel, high molecular weight isoolefin multiolefin copolymers, in particular to halogenated, low-gel, high molecular weight isoolefin diene copolymers, halogenated, low-gel, high molecular weight butyl rubbers, and halogenated, low-gel, high molecular weight isoolefin copolymers synthesized from isobutene, isoprene and optionally further monomers, with a multiolefin content of greater than 2.5 mol %, a molecular weight Mw of greater than 240 kg/mol and a gel content of less than 1.2 wt. % characterized in that said halogenated copolymers are produced in a process comprising the following steps:
a) polymerizing at least one isoolefin, at least one multiolefin, preferably a diene, and optionally further monomers in presence of a catalyst and a organic nitro compound
b) contacting the resulting copolymer under halogenation conditions with at least one halogenating agent.